Aim: Exclusive breastfeeding is recommended for the first 6 months of life, but the breastfeeding rate in premature infants is low. We examined the effect of oral stimulation on infant's strength of suction and the relation between this intra-oral vacuum and breastfeeding duration. Method: Between 2016 and 2018, 211 infants in a Danish neonatal unit were randomised 1:1 and of these 108 to oral stimulation intervention and 103 to control. Suction was measured as peak vacuum at enrolment and a corrected age of 6 weeks. Breastfeeding duration was registered. Results: Vacuum increased from enrolment to a corrected age of 6 weeks in all infants, and no effect of oral stimulation intervention was demonstrated P =.08. Infants born ≤32 gestational weeks had lower vacuum compared with infants born after, 350 vs 398 mbar P <.001. For infants born after 32 gestational weeks, the odds ratio for exclusive breastfeeding at 6 months was 1.99 per 100 mbar increase in vacuum P =.01. Conclusion: In our study, infant's intra-oral vacuum increased with age and was not affected by the oral stimulation intervention. For infants born after 32 gestational weeks, the exclusive breastfeeding rate was positively associated with a strong vacuum.